Three years of MAX-DOAS Observations of Glyoxal and Formaldehyde in Thessaloniki, Greece

We report the first series of ground-based observations of glyoxal (CHOCHO) and formaldehyde (HCHO) over Thessaloniki, Greece during 2022–2025, based on Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS). Vertical column densities and profiles were retrieved with the MMF inversion algorithm, showing good agreement between observations and forward model simulations. Both species exhibit pronounced seasonal cycles, with summer maxima (∼4–6 × 10 14 molec. cm -2 for CHOCHO and ∼1.5–2 × 10 16 molec. cm -2 for HCHO), reflecting the influence of VOC precursor emissions and photochemistry, and winter minima. Enhanced near-surface abundances during summer indicate strong local production with small differences among observations made at different azimuth angles, while low winter levels are consistent with reduced photochemical activity. A significant enhancement of both species was observed during the August 2023 wildfires, highlighting biomass burning as a major episodic source. A weak correlation between the two species is found in winter but stronger in summer, particularly in daily mean observations. Lower values in summer and higher in winter were detected for the CHOCHO to HCHO ratio. This new dataset of CHOCHO and HCHO in Thessaloniki will facilitate the investigation of the origin of emissions sources of these two species, the assessment of local air quality and the validation of satellite retrievals.